Category: Astronomy: Planetary Science

Eclipse Seasons

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

“C’mon in, Sy.”

“Morning, Cathleen. You know my niece Teena.”

“Hi, Teena. What brings you here to my office?”

“I’m working on a school project about eclipses, Dr O’Meara, and I noticed something weird. Uncle Sy said you could explain it to me. You know how an eclipse isn’t in just one place, the Moon writes its shadow along a track?”

Image from GreatAmericanEclipse.com

“Of course, dear, I do teach Astronomy.”

“Sorry, I was just giving context.” <Cathleen and I give each other a look.> “Anyhow, I found this picture of lots of eclipse tracks and see how they weave together almost like cloth?”

“Oh, it’s better than that, Teena. Look at the dates. Is there a pattern there, too?”

“Oooh, the Springtime ones go northeast and the Fall ones go southeast. Hey, I don’t see any in the Summer or Winter! Why is that?”

“It’s complicated, because it’s the result of several kinds of motion all going on at once. Have you ever played with a gyroscope?”

“Uh-huh, Uncle Sy gave me one for my birthday last year. He said that 10 years was old enough I could make it spin without hitting someone’s eye with the string. He was mostly right and I promise I really wasn’t aiming at Brian.”

<another look> “Well … okay. What’s a gyroscope’s special thing?”

“Once you start it spinning it tries to stay pointing in the same direction, except mine acts dizzy a little. Uncle Sy says the really good ones they put in satellites don’t get hardly get dizzy at all.”

“Good, you know gyroscope behavior. Planets spin, too, though a lot slower than your gyroscope. Do you know about planets?”

“Oh yes, when I was small and we looked at the eclipse my Mom and Uncle Sy explained about how we live on a planet that goes round the Sun and sometimes the Moon gets in the way and makes a shadow on us but when the Earth turns so we’re facing away from the Sun we’re in Earth’s shadow.”

“Nice. Well, here’s a diagram about how eclipses happen. It shows four Earth‑images at special points in its orbit. Each Earth has Moon‑images at two special points in the Moon’s orbit. There’s also an arrow coming out of each Earth’s North Pole to show the axis that the Earth spins on. We’ve got three circular motions and each one acts like your gyroscope.”

Adapted from a graphic by Nela, licensed under CCA-SA 4.0

“Does the Moon spin, too?”

“We talked about this a couple years ago, sweetie. The Moon always keeps one face towards the Earth so it spins once each month as it orbits around the Earth. Dr O’Meara’s just using a single circle to cover both, okay?”

“Okay. So there’s three gyroscopes, four really but one’s hiding. The picture says that all three point in different directions, right, and they stay that way?”

“Perfect.”

“Excuse me, but those angles don’t look right. The Earth axis is pointed too close or something.”

“Sharp, Sy. You’re partially correct. Actually, that axis is at a proper 23° angle from the perpendicular to Earth’s orbital plane. It’s the lunar orbital plane and its axis that are off. They’re supposed to be at a 5° angle to Earth’s plane but they’re drawn at 15° to highlight that important line where the two planes meet. The gyroscopes keep that line steady all year.”

“What’s so important about the line?”

“If the Moon is too far above or below Earth’s plane, its shadow is too far above or below Earth to make an eclipse. Eclipses only happen when the line runs through the Sun AND when the Moon is close to the line. The line only runs through the Sun in the Spring and Fall, in this century anyway, so those are our eclipse seasons.”

“Why not every century?”

“A century ago, the eclipses came a few months earlier. The gyroscopes slowly drag the line around Earth’s solar orbit, shifting when the eclipse seasons arrive. If you want a New Year eclipse you’ll have to wait a long, long time.”

~~ Rich Olcott

  • Thanks to Naomi Pequette, Peak Nova Solutions, whose “Eclipses” presentation inspired this post.

Big Spin May Make Littler Spins

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

“Sorry, Vinnie, if there’s anything to your ‘Big Skip‘ idea you can’t blame Jupiter’s Great Red Spot on Io.”

“Come again, Cathleen? Both you and Sy were acting intrigued.”

“That was before I looked up a few numbers. You suggested that a long‑ago grazing collision between Io and Jupiter could account for Jupiter’s weird off‑center magnetic field, its Great Red Spot and Io’s heat and paltry waterless atmosphere. The problem is, there’s two big pieces of evidence against you. The first is Io’s orbit. It’s almost a perfect circle, eccentricity 0.0041, less than half the average of the other Galilean moons. A true circle has zero eccentricity compared to a parabola at 1.0.”

“So why is that evidence against the idea?”

“There’s virtually zero probability that a chaotic skip would send Io directly into such a perfect orbit. Okay, repetitive tugs from Ganymede’s and Europa’s gravity fields could conceivably have acted together to circularize and synchronize Io’s behavior but that would take millions of years.”

“So it’d take a while. Who’s in a hurry?”

“Your idea is, because of the second piece of evidence. Jupiter is a fluid planet, gaseous‑fluid much of the way in, liquid‑fluid most of the rest, right? Lots of up‑and‑down circulation due to outward heat flow from Jupiter’s core, plus twisty Coriolis winds at all levels powered by the planet’s rotation. All that commotion would smear out any trace of your grazing collision, probably within a hundred thousand years. The scars from Shoemaker-Levy’s impact on Jupiter were gone within months. Circularization’s too slow, smearing’s too fast, idea’s pfft.”

“Oh well, another beautiful picture bites the dust.” Vinnie glances up and to the left, the thing he does when he’s visualizing stuff. (On him, a quick glance up and to the right is a bluff tell but he knows I know which makes things interesting.) “Okay, so we’re thinking about how Jupiter’s weird atmosphere and how its equator rotates faster than its poles. That cylinder spinning inside a spinning cylinder idea looks nice for an explanation but I can think of a different way it could happen. How about like a roller bearing?”

“Hmm?”

“Big spinning columns deep inside all around the planet. Think about what goes on in between those cylinders you talked about — two layers flowing at different speeds right next to each other. There’s gonna be all kinds of watchacallit – turbulence – in there, trying to match things up but it can’t. Sooner or later twisters are gonna grow up to be north‑south columns.”

“He’s got a point, Cathleen. His columns would reduce between‑layer friction at the cost of increased between‑column friction. Depending on conditions that could give a lower‑energy, more stable configuration.”

“Spoken like a true physicist, Sy. Columns may be part of the story, but not all of it. There’s mostly‑for evidence but also really‑against evidence.”

Adapted from images by NASA/JPL-Caltech/SwRI/ASI/INAF/JIRAM

“Give us the ‘mostly‑for,’ make us feel good.”

“You guys.” <drawing tablet from her purse, tapping screen> “Alright, here’s a couple of images that Juno sent us when it orbited over Jupiter’s poles.”

“Sure looks like what was in my mind. I’ve seen that before somewhere…. Yeah, Al had that poster up behind his cash register like five years ago.”

“Impressive memory, Vinnie. Anyway, those vortices are similar to your idea, except look at these images critically.”

“Wait, different whirlpool counts top and bottom.”

“Right. These columns obviously don’t go all the way through. They must extend only partway inward until they’re blocked at some lower level.”

“Why can’t I have my columns all the way through if they’re outside the blocking level?”

“You could and there may be something like that inside the Sun, but that’s probably not the case for Jupiter.”

“Why not?”

“That’s the ‘really‑against’ evidence — the Great Red Spot and Jupiter’s off‑center magnetism. Something’s powerful enough to cause those two massive phenomena. That something would disrupt your ring‑in‑a‑ring rotation, at least down to the level where the disrupter lives. Your columns could only operate in some layer deeper than the disrupter’s level but above whatever’s blocking the polar columns. If there is such a layer.”

“Geez. Well, a guy can still hope.”

“But that’s not Science.”

~~ Rich Olcott

Three Feet High And Rising

On By rolcottIn Astronomy: Planetary Science, General: Serious Stuff, Uncategorized1 Comment

“Bless you, Al, for your air conditioning and your iced coffee.”

“Hiya, Susan. Yeah, you guys do look a little warm. What’ll you have, Sy and Mr Feder?”

“Just my usual mug of mud, Al, and a strawberry scone. Put Susan’s and my orders on Mr Feder’s tab, he’s been asking us questions.”

“Oh? Well, I suppose, but in that case I get another question. Cold brew for me, Al, with ice and put a shot of vanilla in there.”

“So what’s your question?”

“Is sea level rising or not? I got this cousin he keeps sending me proofs it ain’t but I’m reading how NYC’s talking big bucks to build sea walls around Manhattan and everything. Sounds like a big boondoggle.” <pulling a crumpled piece of paper from his pocket and smoothing it out a little> “Here’s something he’s sent me a couple times.”

“That’s bogus, Mr Feder. They don’t tell us moon phase or time of day for either photo. We can’t evaluate the claim without that information. The 28‑day lunar tidal cycle and the 24‑hour solar cycle can reinforce or cancel each other. Either picture could be a spring tide or a neap tide or anything in‑between. That’s a difference of two meters or more.”

“Sy. the meme’s own pictures belie its claim. Look close at the base of the tower. The water in the new picture covers that sloping part of the base that was completely above the surface in the old photo. A zero centimeter rise, my left foot.”

“Good point, Susan. Mind if I join the conversation from a geologist’s perspective? And yes, we have lots of independent data sources that show sea levels are rising in general.”

“Dive right in, Kareem, but I thought you were an old‑rocks guy.”

“I am, but I study old rocks to learn about the rise and fall of land masses. Sea level variation is an important part of that story. It’s way more complicated than what that photo pretends to deny.”

“Okay, I get that tides go up and down so you average ’em out over a day, right? What’s so hard?”

“Your average will be invalid two weeks later, Mr Feder, like Sy said. To suppress the the Sun’s and Moon’s cyclic variations you’d have to take data for a full year, at least, although a decade would be better.”

“I thought they went like clockwork.”

“They do, mostly, but the Earth doesn’t. There’s several kinds of wobbles, a few of which may recently have changed because Eurasia weighs less.”

“Huh?”
 ”Huh?”
  ”Huh?”

“Mm-hm, its continental interior is drying out, water fleeing the soil and going everywhere else. That’s 10% of the planet’s surface area, all in the Northern hemisphere. Redistributing so much water to the Southern hemisphere’s oceans changes the balance. The world will spin different. Besides, the sea’s not all that level.”

“Sea level’s not level?”

“Nope. Surely you’ve sloshed water in a sink or bathtub. The sea sloshes, too, counterclockwise. Galileo thought sloshing completely accounted for tides, but that was before Newton showed that the Moon’s gravity drives them. NASA used satellite data to build a fascinating video of sea height all over the world. The sea on one side of New Zealand is always about 2 meters higher than on the opposite side but the peak tide rotates. Then there’s storm surges, tsunamis, seiche resonances from coastal and seafloor terrain, gravitational irregularities, lots of local effects.”

Adapted from a video by NASA’s Scientific Visualization Studio

Susan, a chemist trained to consider conservation of mass, perks up. “Wait. Greenland and Antarctica are both melting, too. That water plus Eurasia’s has to raise sea level.”

“Not so much. Yes, the melting frees up water mass that had been locked up as land-bound ice. But on the other hand, it also counteracts sea rise’s major driver.”

“Which is?”

“Expansion of hot water. I did a quick calculation. The Mediterranean Sea averages 1500 meters deep and about 15°C in the wintertime. Suppose it all warms up to 35°C. Its sea level would rise by about 3.3 meters, that’s 10 feet! Unfortunately, not much of Greenland’s chilly outflow will get past the Straits of Gibraltar.”

~~ Rich Olcott

Loud Enough Was Good Enough

On By rolcottIn Astronomy: Planetary Science, Space Travel, UncategorizedLeave a comment

“Okay, Moire, enough with the strings. I got another question.”

“Of course you do, Mr Feder, but step along more quickly, please. In this heat the sooner I get back to the air conditioning the better I’ll like it.”

“Alright,” <puffing> “why all this fuss about the Voyager 2 spacecraft missing its target by two degrees? Earth’s pretty big, two degrees I can barely see on a protractor. Should be an easy hit.”

“Can you see the Moon?”

“Sure, if there’s no clouds in front of it. Sometimes even in the daytime.”

“A full Moon is only half a degree wide, ¼ of your two degrees.”

“No!”

“Yes.”

“But when it’s just rising it’s huge, takes up half the sky.”

“Check that carefully some evening. Hold up your hand at arm’s length. Your little finger’s about one degree wide. The Moon will be half as wide as that no matter where it is in the sky, we’ve talked about this. You can see half a degree easy and probably a lot less than that. Tycho Brahe, the last great pre‑telescope astronomer, was able to make measurements as small as 1/150 of a degree.”

“Okay, I guess two degrees is a little bigger than I was thinking. But still, Earth’s pretty big, there’s no excuse for Voyager 2 missing it by two degrees.”

“A two‑degree angle is huge when it extends across astronomical distances.” <drawing Old Reliable from its holster, tapping screen> “From Voyager 2‘s perspective at 12 billion miles out the short leg of a two‑degree isosceles triangle spans 419 million miles. That’s over twice the width of Earth’s orbit! Poor Voyager could be pointing past Mars away from us.”

“Big distances from a small angle make a long triangle, got it. What did NASA have to do to get things pointed right again?”

“I consider it a technological miracle. At Voyager‘s distance, Earth’s 8000‑mile diameter spans only 70 milliarcseconds. And before you ask, a milliarcsecond is a thousandth of 1/60 of 1/60 of a degree, about 3 billionths of the way across your little finger. Pretty darn small. Frankly, I’m amazed that Voyager 2 has been able to keep its antenna pointed at us so accurately for so long using tech that dates back to the mid‑70s and earlier. Our tax dollars working hard.”

“Amazing, yeah — something like that’s gotta have a kajillion moving parts. A lubrication nightmare in space I bet.”

“Not as many as you might think. The only parts that move on purpose are small things like its gyroscopes, its tracking optics and the valves on its attitude‑adjustment thrusters.”

“Wait, how’d they point the antenna towards us in the first place? I figured that was on gears.”

“Way too much play in a gear train for this level of accuracy. No, the antenna’s solidly fixed to the rest of the structure. Voyager 2‘s Attitude and Articulation Control System adjusts the whole probe as a unit using propellant bursts through its choice of little thrusters. The mass of a single burst is so small compared to the spacecraft mass that the AACS can manage milliarcsecond‑level orientation control.”

“I heard they finally got it talking to us again. How’d they manage that if it was pointed the wrong way?”

“The key is it was only mostly pointing the wrong way.”

“Like the guy’s ‘mostly dead’ in Princess Bride?”

“Mr Feder, you know that movie?”

“Hey, it’s got the greatest sword fight ever, plus the two‑cups poison challenge and the part where the pirate keeps insulting the prince. What’s not to like? Whaddaya mean, mostly the wrong way?”

“Voyager 2‘s antenna is parabolic, the best shape for transmitting a tight beam. Best doesn’t mean perfect — 50% of the beam’s power stays within a degree or so either side of the center but the rest leaks out to the sides. The same pattern applies to signal reception. Optimal reception happens when the antenna is pointing right at you. If it’s aimed off‑center, reception is worse. Our normal transmission power level wasn’t high enough to punch though the two-degree offset penalty but NASA’s extra-high-power ‘shout’ worked.”

“Caught the flash outta the corner of its eye, huh?”

~~ Rich Olcott

The Big Skip?

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

Suddenly Vinnie gets a grin all over his face. “Tell me something, Cathleen. Suppose I’m a pilot in a shuttle craft like in Star Trek. Tell me how conditions change as I dive down into Jupiter.”

“Hmm .. okay. Mind you, it’ll be a dangerous flight. You’ll fly through an atmosphere that’s mostly molecular hydrogen which is notorious for sneaking into metallic materials and weakening them. I recommend investing in a Starfleet‑grade force shield to keep the atmosphere completely away from your hull. While you’re in the stratosphere high above the cloud decks you’ll see a deep blue sky pretty much the same as Earth’s stratosphere. Try to avoid the thin gray clouds in the upper troposphere — their greasy hydrocarbons will fog your windshield. You want to stick to clear air as much as possible so dodge around the white ammonia‑ice zones. You can drop a couple hundred kilometers more before you hit the top of a brownish ammonium sulfides band.”

“Once I’m that deep there’s clear air underneath the white deck, right?”

“We just don’t know. Unlikely, but if you do want to fly beneath a zone you’ll have to traverse the jetstream separating it from your band. Pick the pole‑ward zone — jetstreams on that side seem to host fewer thunderstorms. Strap in for the jump, because the jetstreams sustain windspeeds 2‑3 times what we get in a Category 5 hurricane. Things’ll get muddier when you drop beneath the brown clouds.”

“Brown as mud, uh-huh.”

“No, I mean literal mud, maybe. First there’s a water‑ice layer and below that there may be a layer of clay‑ish or silicate droplets which may include water of crystallization. I like to visualize clouds of opal, but of course there’d be no sunlight to see them by. A bit lower and you’ll fly through helium rain. Get past all that and you’re about 20% of the way down, about two Earth diameters.”

“That’s where I bump into something?”

“No, that’s the transition zone where heat and pressure convert molecular H₂ into a metallic fluid of protons embedded in a conducting ocean of electrons. Sy, how do you suppose that would affect Vinnie’s aerodynamics?”

“Destructively. If his shuttle’s skin doesn’t rupture he’d be floating rather than sinking. Net density of an intact hull and everything inside would be less than the prevailing density outside where protons are crammed together. Even powered descent would be tough.”

“Sy, that’s exactly what my crazy idea needs! Cathleen, when’s your next Crazy Theory seminar?”

“Not until next term, some time in the Fall. C’mon, Vinnie, out with it!”

Magnetism and wind map by NASA/JPL-Caltech/SwRI/John E. Connerney. Great Red Spot image added by the author.

“All right. That diagram you showed us with the red and blue spots in Jupiter’s off‑center magnetic field? It got me thinking. You get magnetism from moving charge, right, and they say Earth’s field comes from swirls in the molten iron deep underneath our crust. Jupiter doesn’t have iron so much, but you say it’s got electrons in liquid metallic hydrogen and that oughta be able to swirl, too. Maybe Jupiter has a shallow major swirl on that one side.”

“And just what do you suggest would cause a swirl like that?”

“Al was talking the other day about ‘the grand tack hypothesis‘ where Jupiter waltzed in across the inner Solar System before it waltzed back out and settled down where it’s at. Suppose while it was waltzing it hit a planetoid, maybe the size of Io. The little guy couldn’t sink and wouldn’t stick because metallic hydrogen’s liquid so it’d skip across the surface and shoot away and maybe became a moon. That’d raise a swirl like I’m talking about. See, on the map a line crossing the line between the magnetic red and blue spots could be the skip path.”

<silence>

“Hey, and the Great Red Spot, see how it’s like opposite to where I guess the hit was, that’d be like a through-planet resonance like on Mars where that Hellas meteor strike is opposite the Tharsis Bulge.”

<long pause>

“I dunno, Cathleen, Io’s so weird, do you suppose…”

“I dunno, Sy. Io has that magnetic bridge to Jupiter…”

~ Rich Olcott

Stripes And Solids

On By rolcottIn Astronomy: Planetary Science, Uncategorized3 Comments

“Any other broad-brush Jupiter averages, Cathleen?”

“How about chemistry, Vinnie? Big picture, 84% of Jupiter’s atoms are hydrogen, 16% are helium.”

“Doesn’t leave much room for asteroids and such that fall in.”

“Less than a percent for all other elements. Helium doesn’t do chemistry, so from a distant chemist’s perspective Jupiter and Saturn both look like a dilute hydrogen‑helium solution of every other element. But the solvent’s not a typical laboratory liquid.”

“Hard to think of a gas as a solvent.”

“True, Sy, but chemistry gets strange under high temperatures and pressures.”

“Hey, I always figured Jupiter to be cold ’cause it’s farther from the Sun than us.”

“Good logic, Vinnie, but Jupiter generates its own heat. That’s one reason its weather is different from ours. Earth gets more than 99% of its energy budget from sunlight, especially in the infrared. There’s year‑long solar heating at low latitudes but only half‑years of that near the Poles. The imbalance is behind the temperature disparities that drive our prevailing weather patterns.”

“Jupiter’s not like that?”

“Nope. It gets 30 times less energy from the Sun than Earth does and actually gives off more heat than it receives. Its poles and equator are at virtually the same chilly temperature. There’s a small amount of heat flow from equator to poles, but most of Jupiter’s heat migrates spherically from a 24,000 K fever near its core to its outer layers.”

“What could generate all that heat?”

“Probably several contributors. The dominant one is gravitational potential energy from everything falling inward and banging into everything else. Random rock or atom collisions generate heat. Entropy rules.”

“Sounds reasonable. What’s another?”

“Radioactives. Half of Earth’s internal heating comes from gravity, same mechanism as Jupiter though on a smaller scale. The rest comes from unstable isotopes like uranium, thorium and potassium‑40. Also aluminum‑26, back in the early years, but that’s all gone now. Jupiter undoubtedly ate from the same dinner table. Those fissionable atoms split and release heat whenever they feel like it whether or not they’re collected in one place like in a reactor or bomb. Whatever the origin, Jupiter ferries that heat to the surface and dumps it as infrared radiation.”

“Yeah or else it’d explode or something.”

“Mm-hm. The question is, what are the heat‑carrying channels? They must thread their way through the planet’s structure.”

“It’s just a big ball of gas, how can it have structure?”

“I can help with that, Vinnie. Remember a few years back I wrote about high‑pressure chemistry? Hydrogen gets weird at a million bars‑‑‑”

“Anyone’d get weird after that many bars, Sy.” <heh, heh>

“Ha ha, Vinnie. A bar is pressure equal to one Earth atmosphere. Pressures deep inside Jupiter get into hundreds of megabars. Hydrogen molecules down there are crammed so close together that their electron clouds merge and you have a collection of protons floating in a sea of electron charge. They call it metallic hydrogen, but it’s fluid like mercury, not crystalline. Cathleen, when you refer to Jupiter’s structure you’re thinking layers?”

“That’s right, Sy, but the layers may or may not be arranged like Earth’s crust, mantle, core scheme. A lot of the Juno data is consistent with that — a shell of the atmosphere we see, surrounding a thick layer of increasingly compressed hydrogen‑helium over a core of heavy stuff suspended in metallic hydrogen. About 20% down we think the helium is squeezed out and falls like rain, only to evaporate again at a lower level. The core’s metallic hydrogen may even be solid despite thousand‑degree temperatures — we just don’t know how hydrogen behaves in that regime.”

“What other kind of layering can there be?”

“Experiments have demonstrated that under the right conditions a rapidly spinning fluid can self-organize into a series of concentric rotating cylinders. Maybe Jupiter and the other gas planets follow that model and the stripes show where the cylinders intersect with gravity’s spherical imperative. Coaxial cylinders would account for the equator and poles rotating at different rates. Juno data indicates that Jupiter’s equatorial zone has more ammonia than the rest of its atmosphere. Maybe between‑cylinder winds trap the ammonia and prevent it from mixing with the next deeper cylinder.”

~ Rich Olcott

Red And Blue Enigmas

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

“All that cloud stuff goes on in Jupiter’s tissue-paper outer layer. What’s the rest of the planet doing, Cathleen?”

“You’re not going to like this, Vinnie, but all we’ve got so far is broad‑brush averages. The Galileo atmosphere probe penetrated less than 0.2% of the way to the center. The good news is that the Juno probe has been sending us oodles of data about Jupiter’s gravity and magnetic fields. That’s great for planet‑wide theorizing, not quite as useful for weather prediction.”

“Can the data explain the Great Red Spot?”

“Well, it ruled out some ideas. Back in the day we thought the Spot was a deep whirlpool opening a view into the interior. Nope. Juno‘s measurements revealed that the Spot is actually a dome rising hundreds of kilometers above the white cloud‑tops. When one window closes, another one opens, I suppose. The fact that the Spot’s a dome says down below there’s an immense energy source lifting the gases above it. We don’t know what it is or why it’s there or how for two centuries it’s mostly held position in a completely fluid environment.”

“Weird. You’d expect something like that at a special location, like at one of the poles, but the Spot isn’t even on the planet’s equator.”

“Right, Sy. Its latitude is 22° south.”

“Hey, that’s the Tropic of Capricorn.”

“Almost, Vinnie, but not relevant. Earth’s two Tropics are at 23½° north and south. If the Earth’s rotational axis were perpendicular to its solar orbit, the Sun’s highest position would always be directly over the Equator. But Earth’s axis is tilted at 23½° to our orbital plane. To see the noon Sun at the zenith you’d have to be 23½° north of the Equator in June, 23½° south of the Equator in December. Jupiter’s rotational axis is tilted, too, but by only 3°. That rules out significant seasonality on Jupiter, but it also says that on Jupiter there’s nothing special about 22° except that it’s where the Spot hangs out.”

“How about longitude?”

“Longitude on Jupiter is an embarrassing topic. Zero longitude on Earth, our Prime Meridian, runs through Greenwich Observatory in London, right? I don’t want to get into the history behind that. On a completely gaseous planet like Jupiter, there’s no stable physical object to tag with a zero. Jupiter’s cloud‑tops rotate faster near its equator than at its poles. Neither rotation syncs with Jupiter’s magnetic field which is like Earth’s except it’s much more intense and it points in the opposite direction. Oh, and it’s offset from the center of the planet and it’s lumpy. For lack of a better alternative, astronomers arbitrarily thumbtacked Jupiter’s Prime Meridian to its magnetic field. They selected the magnetic longitudinal line that pointed directly towards Earth at a particular moment in 1965. Given a good clock and the field’s rate of rotation you can calculate where that line will be at any other time.”

“Sounds like that ephemeris strategy Sy told me about in our elevator adventure. Why’s that embarrassing?”

“Well, back in 1965 the tool of choice for studying Jupiter’s rotating magnetic field was radio spectroscopy. Technology wasn’t as good as we have now and they … didn’t get a completely accurate rate of rotation. We’re stuck with a standard coordinate system where the Prime Meridian slips about 3° every year relative to the magnetic field. Even the Great Red Spot slips a little.”

“Cathleen, I’ve read that Juno uncovered a region of particularly intense magnetic activity they’re calling Jupiter’s Great Blue Spot. Does it have any connection to the Red Spot?”

Magnetism and wind map by NASA/JPL-Caltech/SwRI/John E. Connerney. Great Red Spot image added by the author.

“Probably not, Sy, the Red Spot’s 15° south and 60° east of the Blue. But with Jupiter who knows?”

“Got any other interesting averages?”

“Extreme wind speeds. There’s a jet stream between each pair of Jupiter’s stripes, eastbound on the poleward side of a white zone, westbound in the other side. Look at the zig‑zag graph on this chart. 75 meters/second is 167 miles per hour is a Category 5 hurricane here on Earth. At latitudes near Jupiter’s equator average winds are double that.”

~~ Rich Olcott

Clouds From Both Sides Now

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

I don’t usually see Vinnie in a pensive mood. Moody, occasionally, but there he is at his usual table by the door, staring at the astronomy poster behind Al’s cash register. “Have a scone, Vinnie. What’s on your mind?”

“Thanks, Sy. Welcome back, Cathleen. What’s bugging me is the hard edges on that picture of Jupiter. It looks like those stripes are painted on. Everyone says Jupiter’s not really solid so how come the planet looks so smooth?”

“Cathleen, this is definitely in your astronomer baliwick.”

“I suppose. It’s a matter of scale, Vinnie. The white zones mark updrafts. The whiteness is clouds that rise a couple hundred kilometers above a brownish lower layer. The downdraft belts on either side are transparent enough to let us see the next lower layer. ‘A couple hundred kilometers‘ sounds like a lot, but that’s only a tenth of a percent of Jupiter’s radius. If Jupiter were a foot‑wide ball floating in front of us, the altitude difference would be as thin as a piece of tissue paper. You might be able to feel the ridges and valleys but you’d have a hard time seeing them.”

“But why does the updraft stop so sharp? Is there like a cap on the atmosphere?”

“The clouds stop, but the updrafts don’t. The cloud tops aren’t even close to the top of Jupiter’s atmosphere, any more than Earth clouds reach the top of ours. C’mon, Vinnie, you’re a pilot. Surely you’ve noticed that most thunderheads top out at about the same altitude. Isn’t the sky still blue above them?”

“That’s higher than the planes I fly are cleared for, but I wouldn’t want to get above one anyway. I know a guy who flew over one that was just getting started. He said it’s a bumpy ride but yeah, there’s still kind of a dark blue sky above.”

“All of that makes my point — our atmosphere doesn’t stop at the tropospheric boundary where the clouds do. Beyond that you’ve got another 40‑or‑so kilometers of stratosphere. Jupiter’s the same way, clouds go up only partway. For that matter, Jupiter has at least four separate cloud decks.”

“Wait, Cathleen — four? I know how Earth clouds work. Warm humid air rises, expanding and cooling as it goes. When its temperature falls below the dew point or freezing point, its humidity condenses to water droplets or ice crystals and that’s the cloud. I suppose if that same bucketful of air keeps rising far enough the pressure gets so low the water evaporates again and that’s the top of the cloud. How can that happen multiple times?”

“It doesn’t, Sy. In Jupiter’s complicated atmosphere each deck is formed from a different gas. Top layer is a wispy white hydrocarbon fog. The white zone clouds next down are ices of ammonia, which has to get a lot colder than water before it condenses. Water ice probably has a layer much farther down.”

“What’s the brownish layer?”

“There’s one or maybe two of them, each a complex mixture of ammonium ions with various sulfide species. The variety of colors in there make the visible light spectroscopy an opaque muddle.”

“Hey, if the brownish layers block what we can see, how do we even know lower layers are a thing?”

“Good question, Vinnie. Actually, we can do spectroscopy in the middle infrared. That gives us some clues. We’d hoped that the Galileo mission’s deep‑diver probe would sense the lower layers directly but unfortunately it dove into a hot spot where the upwelling heat messes up the layering. Our last resort is modeling. We have an inventory of lab data on thousands of compounds containing the chemical elements we’ve detected on Jupiter. We also have a pretty good temperature‑pressure profile of the atmosphere from the planet’s stratosphere down nearly to the core. Put the two together and we can paint a broad‑brush picture of what compounds should be stable in what physical state at every altitude.”

“Those ‘broad‑brush‘ and ‘should‘ weasel‑words say you’re working with averages like Einstein didn’t like with quantum mechanics. Those vertical winds mix things up pretty good, I’ll bet.”

“Fair objection, Vinnie, but we do what we can.”

~~ Rich Olcott

Why Is Io Hot, Europa Not?

On By rolcottIn Astronomy: Planetary Science, UncategorizedLeave a comment

The Acme Pizza and Science Society is back in session at Eddie’s circular table. Al won the last pot so he gets to pick the next topic. “I been reading about Jupiter’s weird moon Io.”

“How’s it any weirder than Ganymede that’s bigger than Mercury?”
  ”Or Europa that’s got geysers and maybe life?”

“Guys, it’s the only yellow moon in the Solar System. You can’t any weirder than that! We got lots of stony moons that are mostly gray, a few water‑ice moons that are white like snow and then there’s Io by itself covered with sulfur.”

“Yellow?”

“Mostly yellow, except where it’s red or dark brown. Or white. They’re all sulfur colors.”

“I’ve seen yellow sulfur, but red?”

“It’s like carbon can be diamond or graphite. Sulfur can be different colors depending on how hot it was when it froze. The article said the white’s probably frozen sulfur dioxide that smells like burning matches.”

“Where’d all that sulfur come from?”

“From inside Io. It’s got like 400 volcanoes that blast out sulfur and stuff. Some of it falls back and that’s why Io is yellow, but a lot gets all the way into space. The article said Io loses a tonne per second. Nothin’ else in the Solar System is that active. Or that dense, probably ’cause it blasted away all its light stuff a long time ago. Anyway, I got a theory.”

“Don’t stop there. What’s the theory?”

“Jupiter’s stripes got all those colors, right, and Sy here wrote astronomers think the brownish bands have sulfur. My theory is that Jupiter got its sulfur from Io. Whaddaya think, Sy?”

“Interesting idea.” <drawing Old Reliable from its holster> “We need numbers before we can upgrade that to a conjecture.” <screen‑tapping> “So, how much sulfur does Jupiter have, and how much could Io have supplied? … Ah, here’s a chart to get us started. Says for every million hydrogen atoms in Jupiter’s atmosphere there’s 40 sulfurs. This Wikipedia article says that the planet masses 1.898×1027 kilograms. 76% of that is hydrogen which calculates to … 1.8×1027 grams of sulfur.”

“That’s a lot of sulfur.”

“Mm-hm. Now, using your tonne per second loss rate and guessing it’s 50% sulfur and that’s been going on for ¾ of the system’s life so far, I get that Io may have shed about 5×1022 grams of sulfur. That’s short by 4½ powers of 10. Sorry, Al, Io contributed a little to Jupiter’s sulfur stash but not enough to promote your idea to a conjecture.”

Jim tosses some chips into the pot. “It’s worse than that, Sy. Galileo‘s probe fell into a clear hotspot so it sampled Jupiter’s gaseous atmosphere but it totally missed the sulfur tied up in those brown clouds. Jupiter’s got even more sulfur than your calculation shows. But there’s still an open question.”

“What’s open?”

Animation by WolfmanSF, CC0, via Wikimedia Commons

“The inner three Galilean moons are locked into resonant orbits. Laplace explained how their separate gravitational fields continually nudge each other to stay in sync. A 1979 paper supported that explanation but then claimed that the moon‑moon nudges produced enough tidal friction within Io to power volcanoes.”

“What’s wrong with that?”

“It doesn’t tell us why Io’s the only one hot enough to boil off all its water.”

“Io had water?”

“Probably, long ago. All three share the same orbital plane and probably formed from the same disk of gas and dust. Both Europa and Ganymede are water worlds, covered by kilometers of water ice. Io should be wet or the other two would be dry by now. Something’s different with Io and it’s not inter‑moon gravitation.”

The algebra behind this chart

“Why not?”

“Numbers. Those moon‑moon interactions are measured in microgravities. Such light impulses can synchronize effectively if repeated often enough, but these just aren’t energetic enough to boil a moon. Besides, Europa stays cool even though it feels a lot more action than Io does.”

“You got a theory?”

“A hypothesis. I’m betting on magnetism. Io’s deep in Jupiter’s lumpy magnetic field which must generate eddy currents in Io’s mostly iron core. I think Io heats up like a pot on an induction stove.”

~~ Rich Olcott

The Sky’s The Limit

On By rolcottIn Astronomy: Planetary Science, Space Travel, UncategorizedLeave a comment

Another meeting of the Acme Pizza and Science Society, at our usual big round table in Pizza Eddie’s place on the Acme Building’s second floor. (The table’s also used for after‑hours practical studies of applied statistics, “only don’t tell nobody, okay?“) It’s Eddie’s turn to announce the topic for the evening. “This one’s from my nephew, guys. How high up is the sky on Mars?”

General silence ensues, then Al throws in a chip. “Well, how high up is the sky on Earth?”

Being a pilot, Vinnie’s our aviation expert. “Depends on who’s defining ‘sky‘ and why they did that. I’m thinking ‘the sky’s the limit‘ and for me that’s the highest altitude I can get up to legal‑like. Private prop planes generally stay below 10,000 feet, commercial jets aren’t certified above 43,000 feet, private jets aren’t supposed to go above 51,000 feet.”

Eddie counters. “How about the Concorde? And those military high-flyers?”

“They’re special. The SST has, um, had unique engineering to let it go up to 60,000 feet ’cause they didn’t want sonic boom complaints from ground level. But it don’t fly no more anyhow. I’ve heard that the Air Force’s SR-71 could hit 85,000 feet but it got retired, too.”

Al’s not impressed. “All that’s legal stuff. There’s a helicopter flying on Mars but the FAA don’t make the rules there. What else we got?”

Geologist Kareem swallows his last bite of cheese melt. “How about the top of the troposphere? That’s the lowest layer of our atmosphere, the one where most of our weather and sunset colors happen. If you look at clouds in the sky, they’re inside the troposphere.”

“How high is that?”

“It expands with heating, so the top depends where you’re measuring. At the Equator it can be as high as 18½ kilometers; near a pole in local winter the top squeezes down to 6 kilometers or so. And to your next question — above the troposphere we’ve got the stratosphere that goes up to 50 kilometers. What’s that in feet, Sy?”

<drawing Old Reliable and screen-tapping…> “Says about 31.2 miles or 165,000 feet. Let’s keep things in kilometers from here on, okay?”

“Then you’ve got the mesosphere and the exosphere but the light scattering that gives us a blue sky happens below them so I’d say the sky stops at 50 kilometers.”

Al’s been rummaging through his astronomy magazines. “I read somewhere here that you’re not an astronaut unless you’ve gone past either 80 or 100 kilometers, which is weird with two cut‑offs. Who came up with those?”

Vinnie’s back in. “Who came up with the idea was a guy named von Kármán. One of the many Hungarians who came to the US in the 30s to get away from the Nazis. He did a bunch of advanced aircraft design work, helped found Aerojet and JPL. Anyway, he said the boundary between aeronautics and astronautics is how high you are when the atmosphere gets too thin for wings to keep you up with aerodynamic lift. Beyond that you need rockets or you’re in orbit or you fall down. He had equations and everything. For the Bell X‑2 he figured the threshold was around 52 miles up. What’s that in kilometers, Sy?”

“About 84.”

“So that’s where the 80 comes from. NASA liked that number for their astronauts but the Europeans rounded it up to 100. Politics, I suppose. Do von Kármán’s equations apply to Mars as well as Earth?”

“Now we’re getting somewhere, Vinnie. They do, sort of. It’s complicated, because there’s a four‑way tug‑of‑war going on. Your aircraft has gravity pulling you down, lift and centrifugal force pulling you up. Lift depends on the atmosphere’s density and your vehicle’s configuration. The fourth player is the kicker — frictional heat ruining the craft. Lift, centrifugal force and heating all get stronger with speed. Von Kármán based his calculations on the Bell X‑2’s configuration and heat‑management capabilities. Problem is, we’re not sending an X‑2 to Mars.”

“Can you re‑calibrate his equation to put a virtual X‑2 up there?”

“Hey, guys, I think someone did that. This magazine says the Karman line on Mars is 88 kilometers up.”

“Go tell your nephew, Eddie.”

~~ Rich Olcott